Abstract [en]

Aggregatibacter actinomycetemcomitans is a Gram-negative periodontitis-associated bacterium that expresses a toxin that selectively affects leukocytes. This leukotoxin is encoded by an operon belonging to the core genome of this bacterial species. Variations in the expression of the leukotoxin have been reported, and a well-characterized specific clonal type (JP2) of this bacterium with enhanced leukotoxin expression has been isolated. In particular, the presence of the JP2 genotype significantly increases the risk for the progression of periodontal attachment loss (AL). Based on these findings we hypothesized that variations in the leukotoxicity are linked to disease progression in infected individuals. In the present study, the leukotoxicity of 239 clinical isolates of A. actinomycetemcomitans was analysed with different bioassays, and the genetic peculiarities of the isolates were related to their leukotoxicity based on examination with molecular techniques. The periodontal status of the individuals sampled for the presence of A. actinomycetemcomitans was examined longitudinally, and the importance of the observed variations in leukotoxicity was evaluated in relation to disease progression. Our data show that high leukotoxicity correlates with an enhanced risk for the progression of AL. The JP2 genotype isolates were all highly leukotoxic, while the isolates with an intact leukotoxin promoter (non-JP2 genotypes) showed substantial variation in leukotoxicity. Genetic characterization of the non-JP2 genotype isolates indicated the presence of highly leukotoxic genotypes of serotype b with similarities to the JP2 genotype. Based on these results, we conclude that A. actinomycetemcomitans harbours other highly virulent genotypes besides the previously described JP2 genotype. In addition, the results from the present study further highlight the importance of the leukotoxin as a key virulence factor in aggressive forms of periodontitis.

Höglund Åberg, Carola

Umeå University, Faculty of Medicine, Department of Odontology.

2013 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

Aggregatibacter actinomycetemcomitans is an oral bacterium that is mainly associated with aggressive forms of periodontitis, which most often starts at an early age. Amongst the virulence factors of A. actinomycetemcomitans, two exotoxins, the leukotoxin (LtxA) and the cytolethal distending toxin (Cdt), are suggested to play an important role in the pathogenicity of aggressive periodontitis. There is also a genetic diversity of the different strains of A. actinomycetemcomitans, and a variation in the ability of different strains to express and release exotoxins has been suggested. Of the different genotypes of A. actinomycetemcomitans, the highly leukotoxic JP2 genotype, which is prevalent in individuals of African origin, seems to be the genotype that is most strongly associated with localized aggressive periodontitis.

This thesis is built upon studies of a West African adolescent population. The aim was to study the virulence characteristics of A. actinomycetemcomitans genotypes with a specific focus on the LtxA and the Cdt in relation to the progression of attachment loss (AL). The specific aim was first, to investigate cross-sectionally the presence of the JP2 and non-JP2 genotypes of A. actinomycetemcomitans in relation to the prevalence of AL and then prospectively to assess the progression of AL in a Ghanaian adolescent population. Second, in clinical isolates of A. actinomycetemcomitans obtained from the participants of the study, the serotypes and the virulence characteristics related to the two exotoxins were studied and associated with the progression of AL at the individual level.

In Paper I, based on the study population consisting of 500 adolescents (mean age 13.2 years; SD ±1.5), it was shown that the overall carrier rate of A. actinomycetemcomitans was high (54.4%) and that the presence of this bacterium was associated with AL ≥ 3 mm. The JP2 genotype was prevalent (8.8%) in this population. In Paper II, 397 (79.4%) of the study participants were periodontally examined again at a 2-year follow-up. The presence of the JP2 genotype of A. actinomycetemcomitans in subgingival plaquewas strongly associated with the progression of AL. This study also provided support for an enhanced estimated risk (odds ratio, OR=3.4), though less pronounced, for the progression of AL in individuals positive for the non-JP2 genotypes of A.actinomycetemcomitans.

In Paper III, all three cdt genes (a, b and c) were detected in 79% of the examined A. actinomycetemcomitans isolates, all of which expressed an active toxin. The distribution of the cdt genes showed a serotype-dependent pattern. In particular, the presence of the b serotypes (both JP2 and non-JP2 genotypes) was associated with the disease progression, whereas the expression of Cdt was not particularly related to the disease progression. In Paper IV, it was shown that the presence of of A. actinomycetemcomitans isolates with high leukotoxicity, also those of the non-JP2 genotypes of A. actinomycetemcomitans, were associated with an increased risk of the progression of AL in relation to the reference group. The main proportion of the serotype b isolates was distributed in the category of highly leukotoxic isolates. The analyses of the non-JP2 genotypes of serotype b indicated a diversity linked to the level of leukotoxicity.

In conclusion, A. actinomycetemcomitans in general was associated with the progression of AL. Individuals with an increased risk of developing progression of AL mainly harboured isolates of A. actinomycetemcomitans with a high leukotoxicity, which suggests that the LtxA is an important virulence factor. Of the two exotoxins, the pathogenic potential was mainly associated with the LtxA, while the role of the Cdt is unclear.